Printing of photographs and other images in newspapers, magazines, books and other hardcopy material generally uses a process known as halftoning. In halftone printing of continuous tone black and white images, paper or other reflective hardcopy material is imprinted with a large number of circules or dots of black ink with the area of each dot being proportional to the blackness (i.e., 1-intensity) of a corresponding incremental area of the original photograph or image. Each halftone dot appears at a position that is equidistant from each adjacent potential halftone dot position so that each halftone dot occupies a single resolution cell or element within a rectangular array or grid. Traditionally, the printing plates for halftone printing were prepared by photographing the image to be reproduced through a screen having the desired interstitial spacing or cell size. This process, known as screening, results in a photographic negative (a "halftone screen") which can be utilized in conventional processes for producing printing plates with the halftone screen exhibiting the desired dot size-image intensity relationship and regularity of spacing between dots.
When an image that has been printed by halftoning is viewed from a distance, the eye performs spatial integration so that various regions of the image are perceived as being black, white, or as being of intermediate intensity (i.e., a shade of grey). Thus, it can be recognized that the degree to which halftone printing is perceived as being identical to the original black and white image largely depends upon the halftone dot frequency that is employed, which commonly is referred to as the line frequency or screen ruling. Another important factor is the ability of the printing press to imprint the type of paper or other hardcopy material being employed with halftone dots that correspond in area with the dots of the halftone screen. For example, because of the type of paper employed and the printing presses used, newspaper halftones typically are printed at a line frequency of 60 to 100 lines per inch, while magazine and book halftones typically are printed at a line frequency on the order of 133-150 lines per inch.
Halftoning also has been used for a number of years in reproducing color images on paper or other reflective hardcopy material. Specifically, color halftoning utilizes separate printing plates (and hence separate halftone screens) for three primary colors (cyan, magneta and yellow) and for the color black. As is known in the art, the three primary colors, cyan, magneta and yellow are called the substractive primary colors because white light that is reflected from each of these colors has substracted from it one of the additive primary colors (red, green and blue). More specifically, when a reflective hardcopy surface is coated with cyan ink or pigment, reflected white light (itself the sum of red, green and blue) includes only the colors green and blue. Similarly, magneta absorbs or substracts the color green from impinging white light and yellow absorbs or substracts the color blue. Thus, for example, if color halftone printing techniques are utilized to first imprint paper or another reflective hardcopy material with a closely packed array of cyan dots and then imprint each interstitial region between the cyan dots with magneta dots, impinging white light that is reflected from the material will largely be devoid of both the colors red and blue. Thus, the spatial integration that occurs when the composite reproduction is viewed from a distance results in the perception of the color green with parameters such as the amount of reflective material that is not imprinted, the reflectivity of that material, and both the reflectivity and the purity of the inks or pigments used determining qualities such as the brightness, shade and intensity of the perceived color. Although combining cyan, magneta and yellow theoretically results in the absorption of all three additive primaries (and hence the color black), modern color halftoning includes a black halftone because slight impurities in commercially available cyan, magenta and yellow printing inks, do not allow production of deep black (high densities) with a cyan, magenta and yellow halftoning process. Further, grey shades do not appear of proper shade when only cyan, magenta and yellow halftone dots are used. Thus, without the use of a black halftone, the printing process would not result in faithful reproduction of black objects and the brightness or the intensity of some of the colors appearing in the reproduction often would differ from that of the image being reproduced.
Each of the four halftone screens that is utilized in the four color halftone process is subject to the constraints and considerations mentioned relative to halftone processing of black and white images, including regularity of halftone screen line spacing (i.e., little or no variation in screen ruling) and control of the halftone dot size to adequately reflect the intensity or brightness of the associated color within incremental regions of the image being reproduced. In addition, in color halftoning, the screen angles (i.e., the angle at which the lines of spaced-apart halftone dots intersect the vertical axis of the image being reproduced) must be different for each of the halftones and must be carefully controlled. Specifically, when an image is reproduced by the color halftone process, optical interference occurs between the intersecting lines of halftone dots that are associated with each of the four color halftone screens. Depending upon the particular screen angles employed, these interference patterns (known as moire) can be extremely discernable. In this regard and as is well known in the art of color halftone printing, a subdued and rather pleasing interference pattern that is known as a rosette results when the screen angles for the cyan, magenta and black color separations are offset from one another by 30.degree.. Since the screen angle for the black halftone color separation customarily is 45.degree., a convention has been adopted in which the screen angles for the magenta halftone color separation and the cyan halftone color separation are 75.degree. and 105.degree., respectively. In this convention, the screen angle for the yellow halftone separation is established at 90.degree..
Photographically producing satisfactory halftone screens (also known as color separations) for use in a four color halftone process utilizes techniques similar to the photographic techniques utilized in producing an achromatic screen for black and white halftone reproduction. Specifically, each color separation is obtained by photographing the image to be reproduced through an appropriate color filter and a screen that is oriented to provide the desired screen angle.
Advances in the fields of digital electronics and optical scanning have led to numerous attempts to develop digital imaging systems for four color halftone processing. Basically, in digital signal processing systems for producing halftone color separations, the image to be reproduced is scanned with an optical scanner to generate digitized signals representative of the color contained in small incremental regions ("pixels") of the image being processed. The digitized color representative signals are then processed to generate digitally encoded signals that are representative of the cyan, yellow, magenta and black components for each of the image pixels. Further processing of the digitally encoded signals is performed to establish a set or array of digitally encoded signals that establish the position, size and shape of the halftone dots that are produced when the data is coupled to a conventional digital output device such as a laser printer or a similar digital output device (e.g., a conventional device that is known as a laser image setter).
Although some previously proposed digital four color halftone processing systems utilize signal processing that allow conventional digital output devices to generate color separations that reasonably approximate color separations obtained by traditional photographic methods, such separations have only been produced by systems that utilize high resolution digital output devices and a substantial amount of digital signal processing. Because of the resulting expense and complexity, commercial application of the previously proposed systems generally has been limited to high volume printing and publishing applications in which the convenience and reliability of a digital four color halftone processing system over the traditional photographic system can be justified. Thus, a need exists for improved four color digital halftoning methods and apparatus that produce screen separations having halftone dots that are formed by a digital output device in a manner that results in satisfactory line separation, screen angle and relationship between halftone dot size and continuous tone intensity.